In copending U.S. application Ser. No. 07/442,653 filed Nov. 29, 1989 for Thermal Seed For Treatment of Tumors and assigned to the same assignee as the present invention, the disclosure which is incorporated herein in its entirety by reference, there is described a thermal seed for implantation in a tumor, for instance a brain tumor, wherein the thermal seed incorporating a ferromagnetic material is excited by an externally developed, varying magnetic field. As discussed in the aforesaid application there are several articles and a patent describing the work of various researchers in the field. Reference is made to Burton, U.S. Pat. No. 3,653,385; Matsuki et al's article in IEEE Transactions on Magnetics, Vol. Mag-21, No. 5, September 1985; Matsuki et al's article in IEEE Transactions on Magnetics, Vol. Mag-23, No. 5, September 1987 and German Patent Application No. P12-84-528.2 of Dr. Forster published Dec. 5, 1968. All of these materials are discussed in some detail in the aforesaid Carter et al. application.
In accordance with the aforesaid Carter et al. application, the length to diameter ratio of the seed should be at least 10 and preferably 20 to minimize demagnetizing effects that reduce the permeability of the ferromagnetic material to a great extent, as much as 8 times. Further the application teaches the use of a conductive coating of bio-compatible material over substantially the entire outer surface of the seed to produce maximum heating of the seed and reduce biocontamination of adjacent tissues.
In the aforesaid device as applied to a practical structure; the larger diameter seed obviously intercepts a larger proportion of the magnetic flux than a small seed but also increases the deleterious demagnetizing effects and deleterious eddy current effects. Thus, with the large diameter seed, it is not possible for the seed to be fully penetrated by the flux and thus is not highly efficient.